Field of the Invention
This invention relates in general to a device and method for non-mechanical laser beam steering, and in particular to a device and method for non-mechanical laser beam steering using holography.
Description of the Related Art
Several non-mechanical beam steering approaches have already been developed. However, these approaches have many limitations that are overcome by this invention.
In 1971 U.S. Pat. No. 3,572,895, entitled “Optical deflection system including an alternating sequence of birefringent prisms and polarizers” and incorporated herein by reference, was issued to Schmidt et al., whose device used birefringent prisms and polarizers to perform light deflection. In this patent, the refracting angles of each polarizer/prism stage by which the incident beam is slightly deflected generally form a geometrical progression with the common ratio 2. Whether or not deflection occurs at any of the factor of 2 stages depends on the controlled polarization of the light for each stage. The resulting angular deflection is essentially set as a binary number and depends on the combination of how much the beam was angularly deflected at each stage. One downside of this approach is that the size of the optics must increase in order to get wider steering angles. In addition, because a polarization approach at multiple stages is used to control deflection, it is impossible for the device to control multiple beams. This deficiency severely limits the level of laser control needed for some applications.
In 1972, a 20 stage digital light beam deflector was designed using Kerr cell birefringence and polarizers, according to Meyer, Rickmann, Schmidt, Schmidt, Rahlff, Shroder and Thust, “Design and performance of a 20-stage digital light beam deflector”, Appl. Opt., vol. 11, no. 8, pp. 1732-1716, August 1972, incorporated herein by reference. Meyer et al disclosed a green laser beam deflected into 1024×1024 raster positions with a deflection rate of 500 kHz. However, this projection system only deflected light approximately +/−11 degrees. In addition, because it also controlled deflection in various stages, it did not have the ability to control multiple laser beams simultaneously.
In 1976, U.S. Pat. No. 3,980,389, entitled “Electro-optical deflection apparatus using holographic grating” and incorporated herein by reference, issued to Huignard et al. and offered a holographic grating approach to increase the angular deflection range for light deflectors. Huignard et al.'s device, required a small angle beam deflector and N holographic diffraction gratings. Each of the N gratings diffracts the light into only one of N directions. The light is angularly multiplexed so that only one of the gratings is capable of diffracting the light into a specific direction. This avoids having to control the polarization through various stages and incrementally obtaining larger angles. However, each desired direction requires a different angularly accessed grating which means it inherently has null regions. Also, since the light must pass through each grating, each one has a potential for energy loss. Since the design must be transmissive and is wavelength dependent, it is difficult if not impossible to use this device for multiband operation.
In 1999, liquid crystal technology was used to form a low-power digital light deflector, using nematic liquid crystal deflectors optimized for wide-angle steering, according to C. M. Titus. P. J. Bos, and O. D. Lavrentovich, “Efficient, accurate liquid crystal digital light deflector,” Proc. SPIE, 1999, vol. 3633, p. 244, incorporated herein by reference. However, this technology consists of multiple stages of liquid crystal wedges for deflection and twisted nematic switches for polarization control. Since several polarization controlling stages are required for steering the laser beam, this approach will not support simultaneous multiple beam control. In addition, liquid crystals are only transmissive in certain spectral ranges and are generally not suitable for multiband operation. For example, they are highly absorptive in the mid-wave infrared which limits their applicability.
In 2004, liquid crystal light deflection was improved by breaking the beam steering into two stages fine angle steering followed by coarse angle steering with an added ability to focus the beam, S. A. Khan and N. A. Riza, “Demonstration of 3-dimensional wide angle laser beam scanner using liquid crystals, Opt. Express, vol. 12, no, 5, pp, 868-882. Mar. 8, 2004. However, as stated before, since multiple polarization controlling stages are required for steering the laser beam, this approach will not support simultaneous multiple beam control.
In 2007, U.S. Pat. No. 7,215,472, entitled “The Wide-angle beam steering system” and incorporated herein by reference, issued to Smith et al. Smith et al.'s device requires sets of azimuth and elevation gratings to get large deflection angles as in U.S. Pat. No. 3,950,359 but attempts to improve on the design by adding several stages of optical phased arrays (“OPA”) which can reduce the null regions. However, the OPAs add more complexity to the multiple stages, are another potential source for loss and may have cross talk. In addition, in order to decrease the null regions, they need to use OPAs to control the deflection at more than one stage which now prevents the device from being able to control multiple beams simultaneously.